Not sure of exact term but I was led to believe it was the trailing arm. It's used in alignment and does have adjustability along it. The bolt in question is the one which actually failed me [another thread] a couple of weeks ago when it broke in half.
The example I mentioned of it loosening was years ago on my RR Sport, not my current LR3. The bold was simply tightened and it was never understood why it loosened. I never had any other trouble with it and all problems immediately went away.
When it was loose there was a sensation that the car would not react to slight turning on the winding freeways quickly enough then all of a sudden would over react [turn tightly]. Turns out that with the loose bolt the sudden reaction was a shifting of inertia coupled with the directional change of the tire causing an near-four-wheel-turning action [though only 3 were changing turned direction].
The bolt in question is of course not shown in this picture as it is the bolt which broke. This image shows the trail repair to get me going again whereby we forced the arm back into a seated position without the bolt in place. So the bolt is what holds this arm to the wheel section from the long thin arm [horizontal].